Mounting process for outgassing-sensitive optics

ABSTRACT

Optics used in a high vacuum environment are mounted by bonding by use of addition polymerizing material which used in that environment. The suitability for use in the high vacuum environment is achieved by precise control of outgassing of trapped and dissolved gases, including low molecular weight hydrocarbons and amines, and unreacted material from component parts of said addition polymerizing material. A plurality of application quantities of the polymer are prepared in a large batch for use as pre-mixed frozen (PMF) material. The use of the large batch enables more precise control of mixture so that near-stoichiometric proportions of the polymer components are easily achieved.

FIELD OF THE INVENTION

This invention relates to addition polymerization. More particularly,the invention relates to the control of outgassing of materials formedaddition polymerization. The invention has particular utility when usedin a mounting process for outgassing-sensitive optics. The invention maybe used for specifying and defining the vacuum quality of polymermaterial used in the fabrication and manufacture of interferometers.

BACKGROUND OF THE INVENTION

Successful mounting of optics has been accomplished by variousmechanical methods. Of these, bonded optic mountings can usually beeffected more quickly and less-expensively than traditional mechanicalclamping methods. The structural adhesives most frequently used to holdoptics to mounts and to bond mechanical parts together are two-partepoxies, urethanes, and room-temperature-vulcanizing (RTV) elastomers.Adhesives emit volatile ingredients during cure or if exposed to vacuumor elevated temperatures. The emitted materials may then condense ascontaminating films on nearby surfaces, such as optics, opto-mechanicalassemblies, or items under process.

For all high vacuum applications, cross-linked polymers are mostpreferred because of extremely high molecular weights generated throughthe chemical reaction. Such polymers do not outgas except for traceamounts of residual components or unreacted low molecular species leftin the polymer network. The two most frequently used polymers areepoxies and silicones. In some instances silicones are preferred overepoxies because of their ability to perform under extreme range oftemperatures; however their adhesive properties with various surfacesare not as good as the epoxies. The epoxies and silicones typically usedin current fabrication processes meet the users' current needs but maynot necessarily meet the outgassing requirements set by the customerfuture generation optic assemblies (e.g., interferometers used in highvacuum environments).

For epoxy resins, proper formulation and mixing of the resin with theamine curing agent in stoichiometric proportions is critically importantto meet the outgassing requirements under vacuum conditions rangingbelow 10⁻⁶ torr. Commonly used amine curing agents under ambientconditions have some vapor pressure and therefore a slight excess orimproper mixing would present serious problems and would not meet thevacuum compatibility and outgassing specifications for the opticassembly. To address this potential issue, some epoxy formulations useamine adducts as curing agents. This approach is ideally suited for highvacuum applications. One distinct advantage of amine adduct curingagents is that it is a product of poly-functional amines andmono-functional epoxy intermediates and have much lower volatility dueto higher molecular weight. As expected, this curing agent produces asuperior vacuum compatibility epoxy end product with low-outgassingperformance.

One area of concern is the outgassing of the materials used for opticassemblies in a vacuum environment. Typically the outgassing speciesinclude the following:

Trapped and dissolved gases, including nitrogen, oxygen and water vapor;

Possible solvents used the cleaning process and adsorbed gases;

Low molecular weight hydrocarbons and amines that were present asimpurities in the epoxy resin and the curing agent; and

residual and unreacted epoxy and amine components.

When building optic assemblies, optic adhesives are typically measuredand mixed as needed. Assemblers are required to dispense an exact amountof the various constituents of two-part epoxies, urethanes, or RTVs.These constituents are then mixed, often in small quantities, thenapplied to the optic for mounting. The problem is getting exact adhesivemixing ratios with this method. And while this method is often adequatefor many standard optic applications, newer applications have very tightrequirements for outgassing and contamination.

SUMMARY OF THE INVENTION

According to the present invention, outgassing of addition polymerizedmaterials is limited by precisely controlling the admixture of theaddition polymerized materials. The present invention achieves asignificant reduction in the amount of material that outgasses from anoptic bond. This, in turn, reduces the amounts of materials that maycondense as contaminating films on critical surfaces. Specifically, bychoosing different bonding agents, the amounts of total mass loss (TML)and volatile condensable materials (VCMs) can be controlled. This isaccomplished by using pre-mixed and frozen (PMF) adhesives instoichiometric proportions for optic bonding.

According to one embodiment of the present invention, PMF adhesives areused in stoichiometric proportions for optic bonding where outgassingand contamination requirements are severe. A representative applicationis the construction of interferometers, which are employed in highvacuum, extreme ultraviolet (EUV) lithography stages. The invention alsoproposes a pass criteria method for accepting or rejecting high vacuum(HV) adhesives and other materials. There are several advantages todoing this:

Stoichiometric mixtures of adhesives are consistently mixed andemployed;

Assemblers do not need to mix adhesives as part of their optic assemblyprocess;

Adhesive volatiles are outgassed in a clean room, away from criticalequipment;

Optic assemblies are conditioned in a temperature-controlled, ultra-highvacuum (UHV) environment;

A pass criteria is available independent of mass, area, or specificvacuum chamber;

Candidate adhesives can be tested and measured using a standard,quantitative method; and

Low-outgassing adhesives can be appropriately chosen for use indemanding HV and EUV environments minimizing risk to customer equipment.

According to the present invention, standard or custom, high-performanceaddition polymerizing adhesives are used in relatively large batches andthe components of the adhesives are pre-mixed in the large batches. Thebatches are then packaged in desired quantities for storage until use.By using larger quantities, constituent measurement errors areminimized. By way of example, a 1 gram error has much less effect on a1000 gram batch than a 10 gram batch. Reducing measurement errors hasbeen shown to reduce the amounts of unwanted outgassing constituents,which typically include hydrocarbons, plasticizers, and silicones. Thedesired quantities are then dispensed into convenient packages (e.g., 5cubic centimeter syringes). To keep the pre-mixed adhesives from curing,the containers are frozen at temperatures ranging from −40 to −80° C.They are preferably kept at this temperature until ready for use.

DESCRIPTION OF THE DRAWING

The drawing FIGURE is a flow chart of an addition polymer procedureaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawing FIGURE, there is depicted a flow chart of anaddition polymer preparation procedure used according to the presentinvention. An addition polymerizing polymer is selected 11 and anapplication quantity is selected 13. A larger batch size is determined15 based on the selected polymer 11 and selected application quantity13. Components of the addition polymerizing polymer are mixed 17 instoichiometric quantities. The desired precision of scoichiometricmixing is used to adapt the larger batch size to the mixing process.This effectively optimizes the batch size according to difficulty inachieving stoichiometric mixing to a desired precision. Thestoichiometric mixing is performed to within predetermined limitscalculated to limit outgasing to acceptable levels. Typically thismixing is to less than ±1% of stoichiometric. The batch is thenseparated 19 into application quantities and inserted 21 intodispensers, such as syringes, tubes or jars. The dispensers are thenchilled sufficiently to retard polymerization for a predetemined shelflife, and retained for storage 25. Typically such chilling is performedto −80° C., although it is possible to use higher temperatures. For atypical polymer, chilling to −40° C. provides a two or three month shelflife and chilling to 0° C. provides a shelf life of approximately oneday.

After storage 25 an application quantity of the polymerizing material isretrieved 31 and applied 35 to the parts for bonding 37, 39. Theassembled parts are cured and outgassed 51 in order to outgas remainingvolitile components and byproducts. After outgassing 51, if no specifictest data is available 61, the assembled components are tested 63 byperforming total material loss (TML) and collected volatile condensablematerials (CVCM) tests. These tests are performed subsequent to saidoutgassing for a given combination of polymer and configuration in theoutgassing-sensitive environment. If the process as applied to theparticular polymer and assembly is deemed satisfactory 69, then theprocedure is qualified; otherwise a different mixture of additionpolymerizing materials is selected or other changes are made to theprocedure.

For an optic assembly to be built for use in a HV environment, its pieceparts must be properly prepared. This involves special fabrication,cleaning, and packaging operations. To assemble a HV optic assembly, anassembler typically collects the prepared parts, along with theadhesives required to assemble them. By using pre-mixed and frozen (PMF)adhesives, the assembler simply goes to the low-temperature freezer andwithdraws the number of syringes needed for the current assembly work.The HV optic assembly process then takes place, but without the need formixing the required adhesives. Once the assembly is complete and properalignment assured, the assembly is left to cure for a period of time(typically one week). This cure time allows the adhesives to evolvevolatiles that are a normal part of the curing process. These volatilesoutgas from the optic assembly in a production clean room, rather thanin a precision optic environment.

The completed optic assemblies can also be further conditioned in a HVenvironment, such as <10⁻⁶ torr. This vacuum environment, along withsome optional heating at 30-40° C. for an extended period of time suchas 5 to 7 days, further increases the outgassing mechanism for theadhesives and other parts of optic assemblies. Once the measured ratesof outgassing are below a specified pass criteria, the HV optic assemblyis deemed suitable for use in an outgassing-sensitive environment. Anexample of equipment used in an outgassing-sensitive environment wouldbe an extreme ultraviolet (EUV) lithography machine.

According to one embodiment of the present invention, pass criteria foroutgassing are based on concepts and terminology defined in ASTM E1559-93, Standard Test Method for Contamination OutgassingCharacteristics of Spacecraft Materials. For every candidate HVadhesive, total mass loss and volatile condensable materials aremeasured on a mass per unit area basis (e.g., microgram/squarecentimeter, mg/cm²) and a mass percentage basis (e.g., microgram/gram,mg/g). The outgassing rate is measured on a mass per unit area per unittime basis (e.g., picogram/square centimeter-second, pg/cm²-s), as wellas a mass percentage per unit time basis (e.g., picogram/gram-second,pg/g-s).

The ASTM E 1559 method typically uses four quartz crystal microbalances(QCMs) to collect material evolved from materials under test. Testresults usually are given in the following categories:

1. Total Mass Loss (TML);

2. Very-high Volatility Condensable Materials (materials <50 atomic massunits (amu));

3. High Volatility Condensable Materials (materials 50-200 amu);

4. Medium Volatility Condensable Materials (materials 200-400 amu);

5. Low Volatility Condensable Materials (materials >400 amu); and

6. Final Outgassing Rate.

TML and final outgassing rate (FOG) are relatively straightforwardmeasured quantities. This is accomplished by operating the QCMs atdifferent temperatures where different species will condense (typically80 K, 160 K, 220 K, and 298 K). The VCMs are measured in amu bands.Atmospheric gases with <50 amu, such as H₂, H₂O, N₂, CO, O₂, CO₂ areconsidered to be harmless to equipment and processes. The VCMs with >50amu will be grouped together for purposes of this pass criterion. If thehigh, medium, and low VCM quantities are summed (either mg/cm² or mg/g),the amounts long-chain molecules, such as hydrocarbons, plasticizers,and silicones, can be bracketed. This measurement, defined as high,medium, & low (HML) VCMs, along with TML and FOG rate can indicate:

how much material an adhesive will lose (TML);

what the constituents of the mass loss will be (i.e., harmlessatmospheric gases or problematic long-chain polymers; HML VCMS); and

how much material come from the adhesive as the outgassing rate tendstowards an asymptote (FOG).

These quantities are used for determining whether an adhesive isappropriate in an outgassing-sensitive environment. There are threeprincipal advantages of this pass criteria method:

1. outgassing results are independent of mass or area;

2. because outgassing results are independent of mass or area,materials, parts, and assemblies can be tested using the same method;and

3. the pass criteria is based on an ASTM standard vacuum chamberconfiguration, allowing different organizations to compare results.

The PMF bonding technique, together with the pass criteria, bringsquantifiable and repeatable methods to critical optic applications. Thisallows properly prepared optic assemblies to be used inoutgassing-sensitive environments without damaging other opticassemblies or items in process.

For high vacuum measuring and testing applications, cross-linkedpolymers are preferred because of extremely high molecular weightgenerated through the chemical reaction. As a result of the highmolecular weight, these polymers do not outgas except for trace amountsof residual components or unreacted low molecular species left in thepolymer network. The two most frequently used polymers in suchapplications are epoxies and silicones. In some instances silicones arepreferred over epoxies because of their ability to perform under extremerange of temperatures; however, their adhesive properties varioussurfaces are not as good as the epoxies. The epoxies and silicones usedin the PMC (polymer matrix composite) fabrication process meet theuser's current needs but may not necessarily meet the outgassingrequirements which may be established for future generationinterferometers.

NASA document, Outgassing Data for Selecting Spacecraft Materials,describes criteria for materials. This criteria is useful in selectingand evaluating vacuum compatible polymeric materials for interferometerapplications in which the materials are held in a vacuum environment.The document citation is, Outgassing Data for Selecting SpacecraftMaterials, William A. Campbell, Jr. and John J. Scialdone, September,1993, Performing Organization Report Number 93E02432;Sponsoring/Mentoring Agency Report Number NASA RP-1124, Revision 3,National Aeronautics and Space Administration Goddard Space FlightCenter, Greenbelt, Md. 20771.

For epoxy resins, proper formulation and mixing of the resin with anamine curing agent in stoichiometric proportions is critically importantin order to meet the outgassing requirements under vacuum conditionsranging below 10⁻⁶ torr. Commonly used amine curing agents under ambientconditions have some vapor pressure and therefore a slight excess orimproper mixing would present serious problem and would not meet thevacuum compatibility and outgassing specifications for the optical tool.To address this potential issue some epoxy formulations use amineadducts as curing agents and is approach is ideally suited for highvacuum applications. One distinct advantage of amine adduct curingagents is that they are a product of polyfunctional amines andmonofunctional epoxy intermediates and have much lower volatility due tohigher molecular weight. Such curing agents produce a superior vacuumcompatibility epoxy end product with low outgassing properties. In orderto meet and exceed vacuum compatibility specifications it is desiredthat epoxy resins, curing agents and outgassing of the final assemblypart be manufactured with the following techniques:

Use premixed epoxies in stoichiometric proportions and frozen to 40° C.The shelf life for most such premixed epoxies at this temperature isabout 6 months. Premixed epoxies are costly but premixing helps toeliminate process variation between batches.

Select an epoxy formulation that uses amine adduct as a curing agent andthe meets this criteria is EPK1C (repackaged under Torrscal/Varian),Dexter Polymer Corporation. This can be a replacement for grey epoxy EPK907. For EPK1C TML is 0.81% and collected volatile condensable materials(CVCM) is 0.02%.

Use an epoxy that is premixed and frozen (PMF) that are vacuumcompatible and meets NASA outgassing specifications. Master Bond sellsseveral PMF epoxies that are vacuum compatible and meets NASA outgassingspecifications. These come with different viscosity and cure times andcertain formulations meet the NASA outgassing specifications.

A significant criteria is to limit outgassing of the materials used inthe assembly process in a vacuum environment. Typically the outgassingspecies include the following:

Trapped and dissolved gases including nitrogen,oxygen and water vapor;

Possible solvents used the cleaning process and adsorbed gases;

Low molecular weight hydrocarbons and amines that were present asimpurities in the epoxy resin and the curing agent; and

residual and unreacted epoxy and amine components; this should not occurif premixed and frozen (PMF) components are used.

Volatile components can be removed by vacuum baking 10⁻⁶ to 10⁻⁶ torr attemperature range between 30 to 40° C. over an extended period of timei.e., 5-7 days. This can be verified by monitoring the outgassingspecies with a GC/MS or MS (without the GC). GC/MS is a very valuabletool for monitoring, evaluating and qualifying the individual elementsor the assembly unit for vacuum compatibility and outgassing selectioncriteria. A suitable GC/MS system can be purchased from AgilentCalifornia Analytical Systems Group. Such monitoring can be used in theestablishment of threshold levels for outgassing for the materials usedin the process.

In addition repeated evacuation and backfilling with dry nitrogen canalso remove volatile components, and this method will help to flush outvolatile impurities very effectively. A detailed procedure for theoutgassing of the parts and final qualification can be developedutilizing dry nitrogen.

The silicone Dow Corning 6-1104 generates methanol as byproduct of thecuring process. This silicone may take several days or weeks tocompletely outgas through a sandwiched interface between the glass andmetal elements. As a result this may not be an ideal candidate for thepresent specification of vacuum compatibility materials. An alternativepolymer Dow Corning 93-500 space grade encapsulant has better outgassingperformance characteristics. This is a two-part system and the curing isachieved by vinyl polymerization. Therefore no volatile components areproduced in the curing processes. This can also be obtained premixed andfrozen (PMF) to achieve optimum results. TML is 0.25% and collectedvolatile condensable materials (CVCM) is 0.05%.

Cleanroom and handling procedures are as follows:

Clean and assemble the parts in a class 100 environment or better;

Use powder free antistatic clean room gloves. Never touch parts orsurfaces with bare hands;

Follow all the cleanroom protocol procedure i.e., bunny suits,facemasks, etc.

Certain materials are considered forbidden materials in a high vacuumenvironment. One such polymer is a type of silicone polymer. Despitethis, some silicone polymers, such as Dow Corning 93-500 silicone may beused in some circumstances. Proper application of silicon polymers iscritical, and silicone, if improperly applied will result in volatilecomponents from the silicone being deposited on the optics asSiO₂/polymer film, thereby degrading optical properties.

One area of concern are incompatible outgassing components in theassembly unit, where specifications have become more critical.Outgassing components from the epoxies must not interact with silicone,optical cements or metals used in the assembly process. This may occurover a period of several months of operation of an interferometer in avacuum environment. Therefore the interaction and chemistry of eachoutgassing component must be understood and eliminated if necessary.

Several moisture and vapor barrier packaging materials are availablefrom Baystat, of Menlo Park, Calif. Preconditioning of the bags in avacuum environment is suggested before it is used to package the tool.Other resources are Assyst SMIF boxes for transporting the tool.

For cured polymers, it is desired to perform TML and CVCM tests afteroutgassing is performed for 5 to 7 days. During this period the polymerwill continue to cure and at the same time outgassing is taking place. Abaseline GC/MS database is established for all components used thevacuum interferometer as tool for process control.

What is claimed is:
 1. Method for selecting and providing polymers foruse in outgassing-sensitive environments, the method comprising:selecting a suitable addition polymerizing material that includes asilicone polymer which achieves curing by vinyl polymerization;determining a quantity of said material for a single application; mixinga quantity of said selected material in a batch of at least four timesthe quantity for the single application, said mixing resulting incombining component parts of said addition polymerizing material forpolymerization, whereby said mixing is provided at stoichiometricproportions within 2% by weight; subdividing the batch into singleapplication quantities; placing a plurality of the applicationquantities in a chilled environment with a temperature between 0° to−80° C., such that polymerization is retarded sufficiently foranticipated future use of the plurality of said application quantitiesas pre-mixed frozen (PMF) material; and providing individual ones of theapplication quantities for use in the outgassing-sensitive environments,thereby permitting cold storage of unused application quantitiesretained for future use while providing said individual ones of theapplication quantities for use as desired.
 2. Method for selecting andproviding polymers for use in outgassing-sensitive environments, themethod comprising: selecting a suitable addition polymerizing material;determining a quantity of said material for a single application; mixinga quantity of said selected material in a batch of at least four timesthe quantity for the single application, said mixing resulting incombining component parts of said addition polymerizing material forpolymerization, whereby said mixing is provided at stoichiometricproportions within 2% by weight; subdividing the batch into singleapplication quantities; placing a plurality of the applicationquantities in a chilled environment with a temperature between 0° C. to−80° C., such that polymerization is retarded sufficiently foranticipated future use of the plurality of said application quantitiesas pre-mixed frozen (PMF) material; providing individual ones of theapplication quantities for use in the outgassing-sensitive environments,thereby permitting cold storage of unused application quantitiesretained for future use while providing said individual ones of theapplication quantities for use as desired; applying one of saidindividual quantities in assembling components for use in anoutgassing-sensitive environment; outgassing the assembled componentsfor at least one day in a high vacuum environment of pressure lower than10⁻⁶ torr and at least 30° C., said outgassing depleting trapped anddissolved gases, including nitrogen, oxygen and water, solvents, if any,used during cleaning processes, low molecular weight hydrocarbons andamines from the component parts of said addition polymerizing material,and residual and unreacted material from said component parts of saidaddition polymerizing material.
 3. Method for assembling optics, themethod comprising: selecting an addition polymerizing polymer suitablefor use in an outgassing-sensitive environment; determining a quantityof said material for a single application; mixing a quantity of saidselected material in a batch of at least four times the quantity for thesingle application, said mixing resulting in combining component partsof said addition polymerizing material for polymerization, whereby saidmixing is provided at stoichiometric proportions within 2% by weight;subdividing the batch into single application quantities; placing aplurality of the application quantities in a chilled environment with atemperature between 0° C. to −80° C., such that polymerization isretarded sufficiently for anticipated future use of the plurality ofsaid application quantities as pre-mixed frozen (PMF) material;providing individual ones of the application quantities for use in theoutgassing-sensitive environments, thereby permitting cold storage ofunused application quantities retained for future use while providingsaid individual ones of the application quantities for use as desired;assembling at least one component of the optics by bonding with said PMFmaterial; applying one of said individual quantities, when ready foruse, to at least one component of the optics to be assembled for use inan outgassing-sensitive environment; outgassing the assembled componentsfor at least one day; and performing total material loss (TML) andcollected volatile condensable materials (CVCM) tests subsequent to saidoutgassing, said TML and CVCM tests performed at least once for a givencombination of polymer and configuration in the outgassing-sensitiveenvironment.
 4. Method for assembling optics, the method comprising:selecting an addition polymerizing polymer suitable for use in anoutgassing-sensitive environment; determining a quantity of saidmaterial for a single application; mixing a quantity of said selectedmaterial in a batch of at least four times the quantity for the singleapplication, said mixing resulting in combining component parts of saidaddition polymerizing material for polymerization, whereby said mixingis provided at stoichiometric proportions within 2% by weight;subdividing the batch into single application quantities; placing aplurality of the application quantities in a chilled environment with atemperature between 0° C. to −80° C., such that polymerization isretarded sufficiently for anticipated future use of the plurality ofsaid application quantities as pre-mixed frozen (PMF) material;providing individual ones of the application quantities for use in theoutgassing-sensitive environments, thereby permitting cold storage ofunused application quantities retained for future use while providingsaid individual ones of the application quantities for use as desired;assembling at least one component of the optics by bonding with said PMFmaterial; and outgassing the assembled components to deplete trapped anddissolved gases, including: nitrogen, oxygen and water, solvents, ifany, used during cleaning processes, low molecular weight hydrocarbonsand amines from the component parts of said addition polymerizingmaterial, and residual and unreacted material from said component partsof said addition polymerizing material.
 5. Method for assembling optics,the method comprising: selecting an addition polymerizing polymersuitable for use in an outgassing-sensitive environment, wherein theaddition polymerizing material includes a silicone polymer whichachieves curing by vinyl polymerization; determining a quantity of saidmaterial for a single application; mixing a quantity of said selectedmaterial in a batch of at least four times the quantity for the singleapplication, said mixing resulting in combining component parts of saidaddition polymerizing material for polymerization, whereby said mixingis provided at stoichiometric proportions within 2% by weight;subdividing the batch into single application quantities; placing aplurality of the application quantities in a chilled environment with atemperature between 0° C. to −80° C., such that polymerization isretarded sufficiently for anticipated future use of the plurality ofsaid application quantities as pre-mixed frozen (PMF) material;providing individual ones of the application quantities for use in theoutgassing-sensitive environments, thereby permitting cold storage ofunused application quantities retained for future use while providingsaid individual ones of the application quantities for use as desired;and assembling at least one component of the optics by bonding with saidPMF material.
 6. Method as described in claim 3, further comprising:using data from said TML and CVCM tests to verify the polymers aresuitable for use in the given configuration in an outgassing-sensitiveenvironment.
 7. Method as described in claim 3, wherein the outgassingis performed in a high vacuum environment of pressure lower than 10⁻⁶torr.
 8. Method as described in claim 7, wherein the outgassing isperformed at a temperature of at least 30° C.
 9. Method as described inclaim 8, wherein the outgassing depletes trapped and dissolved gases,including nitrogen, oxygen and water, solvents, if any, used duringcleaning processes, low molecular weight hydrocarbons and amines fromthe component parts of said addition polymerizing material, and residualand unreacted material from said component parts of said additionpolymerizing material.